Boost creep 911 Turbo

[proffighter]

Hi Fellows

A friend of mine asked me for help with his boost creep issue. We discussed already bit and checked some things already. I try to describe:

- 3.3l Turbo 82
- Precision Turbo 5862 CHRA Ceramic bearings / Trim 0.82
- 964Turbo IC
- K-Jetronic, enrichement by cold start injector (boost steered)
- GHL headers
- Powerhouse dual outlet muffler
- Tial F46P Wastegate (0.7bar spring -> 1.0bar in reality)
- 964 BOV (plastic one)
- electronic boost control (via wastegate ventilation)

Some small other upgrades, but not relevant here I guess, because motor was running fine.

After winter time he went out for a ride, but boost was increasing, at 1.1bar he stopped. For some unknown reason, max boost increased.

Different measurements:

0.4bar spring with wastegate muffler:









He checked and tried a couple of things:

- Tial F46P Wastegate with 0.4bar spring: No change, stock seems even bader
- Boost hose from IC to Tial checked and replaced
- Boost control removed, wastegate ventilation into the open
- Wastegate muffler replaced
- BOV removed and IC port closed
- checked for general leaks (only hearing and watching, no pressure tests)
- Wastegate spring removed and valve block full open: Still about 0.4bar
- SDS ignition

I was not on site yet, but maybe someone has an idea or even better has similar issue. For me it's sound quite strange. What else could have such an effect without changing some components?

All inputs are welcome

BTW Boost pressure reading by stock clock and Zeitronix ZT-2

Seems to lean from 0.7bar up
Seemst to be fine (plateau at 0.4bar) and then loose control and increase, visible on all 3 pics

[David]

Hopefully Chris chimes in soon since he's the guru here. Seems really strange to have boost creep from just sitting with no changes. I can't think of anything you haven't looked at.

I assume when the wastegate spring was removed, the valve moved freely?

[greglepore]

Is it possible something is blocking the inlet to the wastegate? Has the wastegate been off to check?

[TurboKraft]

If I'm understanding correctly from your datalogs:
- 0,4 Feder => 0,36bar @ 2847UPM; 0,49bar @ 4227, 0,98 @ 6018
- 0,8 Feder => 1,1bar and then shut off for safety

Since changing the wastegate spring isn't resulting in lower peak manifold pressure, I recommend:
1. check that the wastegate's control lines are connected correctly -- always review the basics first...
2. get rid of the "wastegate muffler" and repeating the test. Those pipes (resonator / spark arrestor) severely limit exhaust flow out of the wastegate, and we have had them produce significant boost creep. This is also an easy test.

After that, I would normally suggest inspecting the wastegate to see if the valve travel is being limited in some way (obstruction?), or if the diaphragm has a leak (pressure check), but since you just had it apart, I will assume you checked both of these items.


Headers:

The GHL headers have some inherent boost creep because of the unfavorable exhaust flow from the turbo collector into the wastegate pipe. It's a design that is simple to construct and thus low cost.

B&B headers do, too, but inconsistently -- some better and some worse -- because the angle of the wastegate inlet pipe off of the turbo collector is inconsistent. I remember on (EFI) engine that went right past the 11.6psi (0,8bar) spring and made 16psi manifold pressure. It wasn't a problem as that engine was built to run 18psi daily on pump gas, but it was incapable of running *less* maximum boost.

Fabspeed headers also have some boost creep because of the same basic design.